Sea anemones are invertebrates, or animals lacking backbones, in contrast with fishes, which are vertebrates. Over 95% of all the kinds of animals in the world are invertebrates, most of them insects.

Primitive animals, anemones belong to the phylum variously known as Cnidaria (with a silent "c") or Coelenterata ("se-len-ter-a'-ta"). The former name alludes to the cnidae, or nematocysts, that are manufactured by all members of this phylum, and only by them. The latter means "hollow gut," referring to the single body cavity that serves as stomach, lung, intestine, circulatory system, and everything else. There is but one opening (the mouth) into this cavity, through which all water, food, and gametes pass in and out. It is surrounded by few or many tentacles, which are finger-like or filamentous projections, typically studded with nematocysts. They are active in capturing food and transferring it to the mouth, and may be used defensively, too.

Members of Class Anthozoa (which also includes hard and soft corals), sea anemones live attached to firm objects, generally the sea floor, or embedded in its sediments. An anemone's mouth points generally away from the substratum, and is surrounded by relatively short tentacles. Unlike most other anthozoans, sea anemones lack skeletons of any sort and are solitary. Anthozoans such as corals commonly exist as colonies, with many anemone-like individuals attached to one another. Each cylindrical individual is called a polyp. Members of the other three cnidarian classes may exist as polyps, but additionally (or exclusively) as medusae (singular is medusa). A medusa is little more than an upside- down polyp lacking a skeleton, free to swim in the open sea, with somewhat lengthened tentacles -- in short, a jellyfish.

By contrast with their fish symbionts, the 10 host anemones are not all closely related to one another. Belonging to anthozoan Order Actiniaria (hence the term "actinian"), they are members of three different families. The Actiniidae, of which Entacmaea and Macrodactyla are members, is the largest family of sea anemones, and that to which most common, temperate, shore species belong. The exclusively tropical Stichodactylidae, with genera Heteractis and Stichodactyla, is the main host family. Also tropical, Thalassianthidae contains three genera, including Cryptodendrum. Unlike the fish, in which all members and only members of damselfish subfamily Amphiprioninae are symbiotic, most members of families Actiniidae and Thalassianthidae do not participate in symbioses with fishes, and there are also some non-symbiotic stichodactylids.


Nearly all publications on anemone identification are technical. They deal with features such as nature of the animal's muscles, size and distribution of nematocysts, and arrangement of tentacles in relation to internal anatomy. Such characters, which are retained in preserved specimens, require dissection and histological examination to study. They are used partly because most species from the tropics (especially prior to the 20th century) and from deep seas (until the recent advent of submersibles) were originally known only from preserved specimens. We believe actinians can be identified in the field, based on appearance and habitat, although some experts consider nematocyst analysis essential.

A sea anemone is an extremely simple animal. It may be thought of as a cylinder that is closed at both ends. The lower, or pedal, end may be pointed for digging into soft sediments. In anemones of most families, like the host actinians, it is adapted as a pedal disc, which attaches firmly to a solid object like a coral branch or rock (often buried in sediment). In the center of the oral disc, at the opposite, unattached end, is the mouth. Hollow tentacles, arising from the oral disc, surround it. They may be few or many, and arrayed in radial rows or in circlets. Their form is highly diverse -- short or long, thin or thick, pointed or blunt, globular or tree-like. Tentacle number, form, and arrangement are very important in distinguishing genera and species. The cylindrical column (body) of anthozoans is not completely hollow, the name Coelenterata notwithstanding. In sea anemones, vertical partitions (mesenteries) extend from the column wall across the central space part or all of the way to the throat (actinopharynx). Viewed in cross-section, the column therefore resembles a wheel with spokes. Mesenteries also attach on the underside of the oral disc (the radiating lines of attachment may be visible in an animal that is well expanded, has few tentacles, and/or has a thin oral disc), and tentacles arise between them.

In animals with few tentacles, much of the oral disc, the mouth, and sometimes even the upper end of the throat, into which the mouth opens, are visible. The disc can be radially or circularly patterned; the mouth, which can be circular or elongate, may be elevated on a conical projection and may differ in colour from the oral disc.

The column is appropriately tapered to accommodate a pedal and/or oral disc of smaller or greater diameter than itself. In most species of host actinian, the oral disc is much broader than the column. The column, which may be patterned (commonly splotches of colour or longitudinal stripes), can also bear specialized structures along part or all of its length. For example, some tropical anemones (but none that hosts clownfishes) have branched projections from the lower column. Most host actinians have, in the upper part, longitudinal rows of small warts (verrucae; singular is verruca) to which particles of gravel may adhere; commonly these are pigmented differently from the rest of the column.

Sea anemone colour pattern can be important for field identification, but colour itself, being highly variable in most actinians, is of little diagnostic value. Symbiotic algae may affect anemone (as well as coral) colour, either by imparting their own golden brown colour, or by stimulating the animal to produce a pigment that protects the algae against excessive sunlight. Therefore, anemones often blend in with corals and with sand, explaining how such large animals may be so difficult to detect in nature.

Presence or absence of verrucae is a character defining genera. Thus, all species in a particular genus do (e.g. Stichodactyla) or do not (e.g. Entacmaea) have verrucae. Arrangement of tentacles is also important in defining genera. There may be one tentacle per space between mesenteries (so that number of tentacles equals number of mesenteries attaching to the oral disc) or there may be more than one tentacle between each two mesenteries. Members of the family Actiniidae have one tentacle per space. Anemones of families Stichodactylidae and Thalassianthidae can have so many tentacles because up to several, radially arrayed rows of tentacles arise from alternate spaces (the endocoels), whereas only one tentacle arises from the other spaces (exocoels). The single tentacle is positioned at the very edge of the oral disc (margin). This arrangement may be obvious when the animals are well extended.

Keys and descriptions may not work well with captive animals. As explained in chapter 6, aquarium-kept anemones can lose their colour after a remarkably short time, probably because their algae do not thrive under artificial conditions, and tentacle shape may also change. Fish symbionts should not be used to identify anemones in captivity, as they can be in nature, because many fishes can acclimate to most host anemones.


Adhesive Sea Anemone

Original description As Cryptodendrum adhaesivum, from a specimen collected at Koseir in the Red Sea

Other name previously used Stoichactis digitata (by Doumenc 1973)

Diagnostic field characters Tentacles extremely sticky; short (to 5 mm long), dense, of two forms: those in center of oral disc have narrow stalk with five or more short branches at end (i.e. resembling a miniature glove); those near the edge simple elongate bulbs about 1 mm diameter; at extreme margin is a ring of tentacles like the central ones but with fewer branches. Tentacles of the two forms usually different colours: observed combinations include yellow and pink, blue and gray, green and brown; occasionally tentacles of another colour occur in patches amid those of predominant colour.

Details Oral disc to 300 mm diameter, flat when expanded, but commonly undulating. Entirely covered with tentacles except immediately around mouth, which can be fuchsia, yellow, green, white. Moreover, tentacle stalk and tips may differ in colour. Therefore, may be extremely colourful animal.

Similar species Specimens of Stichodactyla are superficially similar, with many, short tentacles. However, the two distinct types of tentacles arrayed in separate fields is a feature unique to C. adhaesivum. Tentacles of other species may adhere, and pull off the anemone; those of C. adhaesivum remain attached to the anemone.

Distribution Australia to southern Japan and Polynesia, Micronesia, and Melanesia westward to Thailand, Maldives, and the Red Sea

Fish A. clarkii


Bulb-Tentacle Sea Anemone

Original description As Actinia quadricolor, from specimens collected in the Red Sea, near Suez

Other names previously used include Gyrostoma heliant[h]us (by Fishelson 1970, Masry 1971, Fricke 1974), G. quadricolor (by Fishelson 1970, Fricke 1974), Physobrachia ramsayi (by Mariscal 1970, Friese 1972, Mariscal 1972, Uchida et al. 1975), P. douglasi (by Allen 1972, 1975, 1978, Ross 1978, Cutress and Arneson 1987), Radianthus gelam (by Allen 1972, 1978, Friese 1972), Cymbactis actinostoloides (by Moyer and Sawyers 1973), Parasicyonis actinostoloides (by Uchida et al. 1975 as P. actinostoroides, Moyer 1976, Moyer and Bell 1976), P. maxima (by Uchida et al. 1975, Moyer 1976, Moyer and Bell 1976)

Diagnostic field characters Each long (to 100 mm) brown tentacle usually with bulb at or somewhat below end; tip of tentacle red (rarely blue), equator of bulb white. Bulb seems to be related to presence of fish, and can disappear; tentacle lacking a bulb has white ring where equator would form. Tentacles without bulbs are blunt-ended. As a rule, in shallow water (e.g. on tops of reefs) polyps small (oral disc diameter 50 mm), clustered together in crevices or adjacent on coral branches, so that tentacles are confluent, forming extensive field; in deep water (e.g. on reef slopes) polyps solitary, large (to 400 mm diameter), with base anchored in deep hole.

Details Animal commonly attached deeply in crevice or hole so that only emergent tentacles visible. Column without verrucae; usually brown, sometimes reddish or greenish; gradually flared from small pedal disc. Oral disc same brown colour as tentacles. Tentacles can collapse when disturbed, appear gray-green. The most numerous host actinian, widespread geographically and abundant locally.

Similar species The smooth column is unique among symbiotic sea anemones, as are the bulbed tentacles.

Distribution Micronesia and Melanesia to East Africa and the Red Sea, and from Australia to Japan

Fish A. akindynos, A. allardi, A. bicinctus, A. chrysopterus, A. clarkii, A. ephippium, A. frenatus, A. mccullochi, A. melanopus (primarily clustered form), A. omanensis, A. rubrocinctus, A. tricinctus, Premnas (solitary form only)


Beaded Sea Anemone

Original description As Actinia aurora, from specimens collected in New Ireland

Other names previously used include Radianthus koseirensis (by Mariscal 1970, 1972), Radianthus simplex (by Allen 1972, Moyer 1976), Bartholomea sp. (by Uchida 1975)

Diagnostic field characters Tentacles to 50 mm long, with swellings (up to 20 on long tentacles) at intervals, either on only one side or nearly surrounding a tentacle so that it resembles a string of beads. Swellings often white. Oral disc broad, to 250 mm or possibly more, spread flat or slightly undulating at surface of sediment.

Details Tentacles brown or purplish, arising from oral disc of the same colour; outermost tentacles may be shorter than inner, and can have purplish or greenish cast. Oral disc mostly visible because of sparse tentacles; may have white or brown radial markings that can continue onto tentacles. Tentacles may be sticky to touch; tapered to point that may be magenta in colour. Adhesive verrucae on upper column lighter in colour than column; lower column often mottled or solid orange or red. Animals attached to buried objects capable of retracting completely into sediment.

Similar species Macrodactyla doreensis, Heteractis malu, and H. crispa also live burrowed into sediment. Tentacles of the other two species of Heteractis may also be magenta-tipped, but those of H. aurora are unique in having swellings at intervals. Tentacles of H. aurora are intermediate in length between those of H. malu (shorter) and H. crispa and M. doreensis (longer). Tentacles in some individuals of H. aurora nearly as sparse as those of H. malu. The column of H. aurora is similar in texture to that of H. malu.

Distribution Micronesia and Melanesia to East Africa and the Red Sea, and Australia to the Ryukyu Islands

Fish A. akindynos, A. allardi, A. bicinctus, A. chrysogaster, A. chrysopterus, A. clarkii, A. tricinctus


Leathery Sea Anemone

Original description As Actinia crispa, from specimens collected in the Red Sea

Other names previously used include Radianthus kuekenthali (by Mariscal 1970, 1972, Uchida et al. 1975, Moyer 1976), R. malu (by Allen 1972, 1973, 1975), R. ritteri (by Allen 1978), H. macrodactylum (by Cutress and Arneson 1987)

Diagnostic field characters Tentacles long (typically to 100 mm), sinuous, evenly tapered to point, often tipped mauve or blue, rarely yellow or green. Oral disc widely flared, may exceed 500 mm diameter, but commonly 200 mm. Column gray in colour, leathery in texture, with prominent adhesive verrucae; lower part rarely mottled with yellow. Column buried in sediment so oral disc lies at surface of sediment, or pedal disc attached to branching coral.

Details Tentacles very numerous -- to 800 counted. Oral disc usually brownish violet or gray, rarely bright green. Tentacles shrivel when animal is disturbed, and assume green or gray luster; may shorten greatly in absence of fish. For an animal attached to coral branches, verrucae adhere to branches, holding oral disc open among them; verrucae adhere to sediment particles if animal lives in sediment.

Similar species Heteractis magnifica rarely lives on branching coral. Its blunt tentacles and brightly coloured column are distinctive. Macrodactyla doreensis, Heteractis aurora, and H. crispa also live burrowed into sediment. In contrast to anemones of those three species, H. crispa has many, long tentacles. Those of the other two burrowing species of Heteractis may also be magenta-tipped. Tentacles of H. crispa may contract in the absence of a fish, but they are more numerous than in H. malu, and lack the swellings of H. aurora. The column of H. crispa is unique among host actinians.

Distribution French Polynesia, Micronesia, and Melanesia to the Red Sea, and Australia to Japan

Fish A. akindynos, A. bicinctus, A. chrysopterus, A. clarkii, A. ephippium, A. latezonatus, A. leucokranos, A. melanopus, A. omanensis, A. percula, A. perideraion, A. polymnus, A. sandaracinos, A. tricinctus


Magnificent Sea Anemone

Original description As Actinia magnifica, from specimens collected at Vanikoro, Santa Cruz Islands, New Hebrides

Other names previously used include Radianthus ritteri (by Mariscal 1970, 1972, Allen and Mariscal 1971, Allen 1972, 1975, 1978), R. paumotensis (by Allen 1972, Friese 1972), R. macrodactylus (by Uchida et al. 1975), R. malu (by Allen 1978), H. ritteri (by Cutress and Arneson 1987)

Diagnostic field characters Typically occupies fully exposed, prominent position, attached to solid object such as coral boulder. Cylindrical column of uniform bright colour (commonly blue, green, red, white, chestnut brown). Oral disc to 1 m diameter (although commonly 300-500 mm), flat to gently undulating, densely covered with finger-like tentacles (to 75 mm long) that hardly taper to blunt or slightly swollen end. May irritate human skin and raise welts.

Details Lower portion of tentacles same colour as oral disc (usually shade of brown), terminal portion yellow, green, or white; some tentacles bifurcate or with side branch. Tentacles approach mouth to within 20-30 mm; central oral disc yellow, brown, or green, often raised so that mouth sits on a cone. Column with longitudinal rows of translucent verrucae same colour as column or slightly lighter or darker. Animal capable of almost complete contraction so that only a tuft of tentacles is visible in center.

In western Indonesia, several small individuals of identical colouration may cluster together, resembling one large animal. Elsewhere (e.g. Maldives, Malaysia, French Polynesia), tens or hundreds of identically coloured individuals form extensive beds; presumably they constitute a clone.

Similar species This is probably the most distinctive and most commonly photographed species of host actinian. Its exposed habitat is unique, as is its brightly coloured, gently flared column. Only Stichodactyla mertensii may exceed it in diameter, but H. magnifica is a much more substantial animal. Its blunt tentacles are unique in the genus; those of S. haddoni are similarly shaped but shorter and more densely arrayed.

Distribution French Polynesia to East Africa, and Australia to the Ryukyu Islands

Fish A. akallopisos, A. akindynos, A. bicinctus, A. chrysogaster, A. chrysopterus, A. clarkii, A. leucokranos, A. melanopus, A. nigripes, A. ocellaris, A. percula, A. perideraion


Delicate Sea Anemone

Original description As Discosoma malu, from specimens collected at Mer, in the Torres Straits

Other names previously used include Macranthea cookei (by Reed 1971), Radianthus papillosa (by Dunn 1974, Moyer 1976), Antheopsis papillosa (by Cutress 1977)

Diagnostic field characters Tentacles sparse, stubby (rarely to 40 mm long), of variable length even within one radial row, commonly magenta-tipped. Oral disc lies at surface of sediment in which delicate column is burrowed. Column commonly pale cream or yellow colour, may have splotches of deep yellow or orange.

Details Tentacles arise from brown or purplish (rarely bright green) oral disc as much as 200 mm in diameter that may have white radial markings; evenly tapered to point or slightly inflated in middle; lower part same colour as oral disc, but upper portion may have several white rings or green end. Column very thin in expansion; upper part violet-brown (due to zooxanthellae) with longitudinal rows of adhesive verrucae. Anemones can retract completely into sediment; most common in shallow, quiet waters.

Similar species Macrodactyla doreensis, Heteractis aurora and H. crispa also live burrowed into sediment. The columns of all four species may have red or yellow blotches; that of H. crispa is much firmer than that of H. malu (which is similar to that of H. aurora). In contrast to animals of the other three species, H. malu has relatively short tentacles, typically of variable length; tentacles of the other two species of Heteractis may also be magenta-tipped. Tentacles of H. crispa may contract in the absence of a fish, but H. malu has fewer tentacles per radial row; tentacles of H. aurora may be similarly sparse, but have swellings at intervals.

Distribution: Scattered localities from the Hawaiian Islands to Australia and northwards to Japan

Fish A. clarkii


Corkscrew Tentacle Sea Anemone

Original description As Actinia doreensis, from specimens collected at Port Dorey, New Guinea, now Manokwari, Irian Jaya, Indonesia

Other names previously used include Macrodactyla gelam (by Mariscal 1972), Radianthus malu (by Moyer 1976), H. gelam (by Cutress and Arneson 1987)

Diagnostic field characters Tentacles few, long (to 175 mm), all alike, sinuous, evenly tapered to point, sometimes (but not invariably) assuming corkscrew shape. Oral disc widely flared, to 500 mm diameter but commonly considerably less, with radial white lines that may extend onto tentacles; lies at surface of sediment. Column buried in sediment; lower part dull orange to brilliant red, upper part brownish with non-adhesive, prominent white round to ovoid (eye-shaped) verrucae in longitudinal rows.

Details Oral disc usually purplish-gray to brown, sometimes with a green cast; tentacles basically same colour but tips may be darker or lighter. When disturbed, tentacles may shrivel or may adhere to collector's hand and pull off. Often found in mud, generally no deeper than 5 m, commonly without fish; can retract completely into sediment.

Similar species Heteractis aurora, H. crispa, and H. malu also live burrowed into sediment, and may have red or yellow pigmentation on the lower column. Tentacles of M. doreensis lack bulges, in contrast to those of H. aurora, are fewer than those of H. crispa, and are longer than those of H. malu. The column of M. doreensis is thin, and has distinctive verrucae. The distribution of this species is the most restricted of any host anemone.

Distribution Japan south to New Guinea and northern Australia

Fish A. chrysogaster, A. clarkii, A. perideraion


Gigantic Sea Anemone

Original description As Priapus giganteus, from specimens collected in the Red Sea

Other names previously used include Discosoma giganteum (by Gohar 1948, Schlichter 1968), Stoichactis kenti (by Mariscal 1969, 1970, 1972; Allen 1972, 1973, 1978; Uchida et al. 1975)

Diagnostic field characters Deeply-folded oral disc (more pronounced with size), covered with short (average 10 mm), slightly tapering tentacles that typically all vibrate constantly. Tentacles extremely sticky in life, adhering to collector's hand, and pulling off in clumps; but do not cause stinging sensation. Typically in such shallow water that animals may be exposed at low tide.

Details May be extraordinarily abundant. Oral disc rarely as much as 500 mm diameter, usually lies at surface of sand, often among corals; pedal disc attached to buried object. Non-adhesive verrucae on upper column blue to maroon, contrasting with yellowish, pinkish, tan, greenish-blue, or gray-green column. Basal portion of each tentacle colour of the oral disc (often tan or pink); colour of bluntly pointed distal part -- which is what is generally noted as tentacle colour -- commonly brown or greenish, rarely a striking purple or pink, deep blue, or bright green. Much of central oral disc bare, but deep oral disc folds may hide mouth.

Similar species The shallow, sandy habitat is unusual. Stichodactyla mertensii, which lives on hard substrata, has a flat oral disc and distinctive column. Stichodactyla haddoni typically lives in cleaner sand and deeper water, and its oral disc folds are more regular and more separate than those of S. gigantea; tentacle form and fish symbionts of the two also clearly separate them.

Distribution Micronesia to the Red Sea, and Australia to the Ryukyu Islands

Fish A. akindynos, A. bicinctus, A. clarkii, A. ocellaris, A. percula, A. perideraion, A. rubrocinctus


Haddon's Sea Anemone

Original description As Discosoma haddoni, from specimens collected on the northern Great Barrier Reef

Other names previously used include Stoichactis kenti (by Friese 1972, Moyer and Sawyers 1973), S. gigantea (by Friese 1972), S. haddoni (by Uchida et al. 1975, Moyer 1976, Moyer and Steene 1979).

Diagnostic field characters Slightly to deeply folded oral disc lies on or above sand surface; tentacles either bulbous or with basal "stalk," at the end of which is a blunt or swollen terminal portion that can appear puckered (on close examination). Exocoelic tentacles more robust than the endocoelic ones with which they alternate. Column sturdy.

Details Oral disc diameter commonly 500 mm, rarely 800 mm; yellowish to orange tentacle-free oral area 10-20 mm in diameter. Oral disc, lower portion of tentacles, and column drab -- commonly yellowish or tan. Tentacle ends can be green, yellow, gray, or rarely pink, which can give oral disc a variegated appearance. Exocoelic tentacles usually white, may be up to twice as long as endocoelic, point outward in well expanded animals. Tentacles sticky to touch, may adhere to human skin so strongly that they pull off the anemone; contact with them is painless but can raise welts. Small, non-adhesive verrucae on uppermost column are same colour as column or light rose to purple. Anemone can pull rapidly and completely beneath the sand when disturbed, leaving its fish to hover over the resulting depression.

Similar species Stichodactyla gigantea also lives in sand but typically in shallower water, and folds of its oral disc are less regular and more closely spaced. The oral disc of S. mertensii, which lives on firm substrata, lies fairly flat. The column of S. haddoni is more substantial than that of either, and its tentacles are longer and distinctively shaped. The other species lack robust exocoelic tentacles. Tentacles pull off of S. gigantea as well, but in massive clumps rather than one or several at a time.

Distribution Fiji Islands to Mauritius, and Australia to the Ryukyu Islands

Fish A. akindynos, A. chrysogaster, A. chrysopterus, A. clarkii, A. polymnus, A. sebae


Mertens' Sea Anemone

Original description As Stichodactyla mertensii, from specimens collected in the easternmost Caroline Islands

Names previously used Stoichactis giganteum (by Mariscal 1970, Allen and Mariscal 1971, Allen 1972, 1973, 1975)

Diagnostic field characters Oral disc to 1 m or even more diameter; tan to white column with longitudinal rows of verrucae pigmented magenta or orange (which appear purplish at depth); non-adhesive tentacles club-shaped to finger-like -- all may be short (10-20 mm long), or some (in patches) very long (to 50 mm or more).

Details This anemone holds the record for oral disc diameter (shape is often more ovoid than circular). Broadly flared oral disc lies smoothly over substratum, following its contours, or undulating slightly, held open by verrucae adhering to underlying coral or rocks, which may be related to this anemone's living only on hard surfaces, often reef slopes. No verrucae below wide upper column, but splotches of pigment continue down short, narrow column in more or less longitudinal streaks. Small pedal disc frequently attached in crevice into which animal can retract (although not rapidly). Tentacles, of uniform diameter, blunt-ended or pointed: short ones all colour of the oral disc, sometimes with narrow white basal portion; long ones may be white-, yellow-, or green-ended. Tan oral disc almost entirely covered with tentacles; yellow or greenish tentacle-free oral area 20-50 mm diameter. Symbiotic fishes may be melanistic.

Similar species Stichodactyla gigantea and S. haddoni live in sand. Their oral discs are wavy, in contrast with that of S. mertensii, and their columns more substantial. The only other host anemone that rivals S. mertensii in size is H. magnifica, which has longer, blunt tentacles and a brightly coloured, cylindrical column.

Distribution Micronesia and Melanesia to East Africa, and Australia to the Ryukyu Islands

Fish A. akallopisos, A. akindynos, A. allardi, A. chrysogaster, A. chrysopterus, A. clarkii, A. fuscocaudatus, A. latifasciatus, A. leucokranos, A. ocellaris, A. sandaracinos, A. tricinctus